This invention relates to a speckle multiplexed hologram recording apparatus and method of the inline type, and more particularly to an inline type speckle multiplexed hologram recording apparatus and method which condenses signal light and reference light on a hologram recording medium without increasing the aperture of a lens.
In recent years, a hologram recording and reproduction system for recording and reproducing a large mount of data utilizing a hologram technique has been proposed. The hologram recording and reproduction system includes a recording system for illuminating signal light including recording data produced by a spatial light modulation section such as, for example, a liquid crystal device and reference light set corresponding to the signal light upon a hologram recording medium (recording member) at predetermined angles to record interference fringes produced by the signal light and reference light in the hologram recording medium. The hologram recording and reproduction system further includes a reproduction system for illuminating reproduction illumination light on the hologram recording medium to produce diffraction light corresponding to the interference fringes recorded in the hologram recording medium and receiving and analyzing the diffraction light by means of a light receiving device such as a CCD image sensor to reproduce the recorded data. It is to be noted that holograms for recording data recorded by illumination of light once is called page.
Further, in the hologram recording and reproduction system, a method called multiplexing recording is used in order to achieve a high recording density. Different from a recording method for a conventional optical disk, the multiplexing recording method records a great number of pages independent of each other at one place. As representative ones of such multiplexing recording methods as just described, an angle multiplexing recording method, a shift multiplexing recording method, a phase code multiplexing recording method and so forth are publicly known, and many other multiplexing recording methods are known. One of such multiplexing recording methods which is called speckle multiplexing (or correlation multiplexing) relates to the present invention and hereinafter described in detail.
Incidentally, where a hologram is recorded in a recording member, there is a problem that an unavailable region arises from the thickness of the hologram. For example, if the thickness is ignored, then the area necessary for recording one page is such a range as shown in FIG. 9. However, since the recording member actually has a thickness, where such an overlap of signal light and reference light in the recording member as shown in FIG. 10 is taken into consideration, the area necessary for recording one page is considerably great when compared with that where the thickness is not taken into consideration. Besides, an unavailable region 40 appears, and this decreases the recording density and the dynamic range of a signal which can be recorded in the recording member.
One of possible countermeasures for solving the problem of the unavailable region is to minimize the angle defined by the signal light and reference light. However, if the angle defined by the signal light and reference light is small, then a phenomenon appears that the Bragg selectivity decreases to decrease the recording density conversely.
The speckle multiplexing method mentioned hereinabove is effective as a multiplexing method which enhances the recording density which decreases if the angle defined by the signal light and reference light is reduced. As seen in FIG. 11, according to the speckle multiplexing method, a diffusion member is placed in an optical path of the reference light so that the reference light diffused by the diffusion member and the signal light interfere with each other in the recording member.
Generally, the reference light diffused at random gives rise to random interference and consequently obtains a random intensity distribution called speckle pattern. In particular, according to the speckle multiplexing method, a speckle pattern and signal light interfere with each other. A hologram recorded in such a manner as just described has a characteristic that, if it is displaced by a distance equal to the speckle size, then an image is not reproduced. This is because, while, in the shift multiplexing which uses normal spherical wave reference light, a distance which depends upon the Bragg selectivity is required as the shift amount, in the speckle multiplexing, the distance corresponding to the speckle size is equal to the shift amount. Where the characteristic just described is taken into consideration, it can be recognized that the speckle multiplexing method is free from the problem that the Bragg selectivity decreases to decrease the recording density even if the angle defined by the signal light and reference light is reduced in order to decrease the unavailable region.
In other words, according to the speckle multiplexing, the signal light is not reproduced when the correlation function of the reference lights used upon recording and upon reproduction is zero. This signifies that, if the reference lights used are so different from each other that the correlation function is zero, then multiplexing recording can be implemented. If this is utilized, then also a speckle multiplexing method seems available which does not depend upon such a movement of a recording member as described. For example, also it is possible for the diffusion member shown in FIG. 11 to move to sufficiently vary the speckle pattern on the recording member to achieve multiplexing recording.
As can be recognized from the foregoing, where the thickness of the recording member is taken into consideration, in order to decrease the unavailable region, it is significant to reduce the angle defined by the signal light and reference light, and the speckle multiplexing method is effective as a method for preventing decrease of the recording density arising from decrease of the Bragg selectivity in this instance. Also a system which adopts such a viewpoint as just described is available. For example, as a system of the type just mentioned, an inline type hologram recording apparatus is disclosed in H. J. Coufal, D. Psaltis and G. T. Sincerbox, “Holographic Data Storage”, ED; Springer; p. 396, Digital Holographic Demonstration System. A configuration of the inline type hologram recording apparatus is shown in FIG. 12.
Referring to FIG. 12, the inline type hologram recording apparatus includes a spatial light modulator (SLM) 21 for intensity modulating laser light 50, and a diffuser 22 disposed around the spatial light modulator 21 for diffusing the laser light 50 at random. The inline type hologram recording apparatus further includes a lens 23 for condensing signal light 100 and reference light 200 on a hologram recording medium 24 so that a hologram is recorded in the hologram recording medium 24. The inline type hologram recording apparatus further includes another lens 25 for condensing diffraction light produced from the hologram recording medium 24 to form an image on an image sensor 26. The image sensor 26 performs a photoelectric conversion process for the image formed thereon to produce a reproduction signal.
Upon recording, the laser light 50 is introduced as parallel light into the spatial light modulator 21 and the diffuser 22. The laser light 50 introduced in the spatial light modulator 21 is converted into signal light 100 whose intensity is modulated with display data. On the other hand, the laser light 50 introduced in the diffuser 22 is diffused at random by the diffuser 22 to form reference light 200 having a speckle pattern. The signal light 100 and the reference light 200 are condensed on the hologram recording medium 24 by the lens 23. In this instance, interference fringes of the signal light 100 and the reference light 200 are recorded in the hologram recording medium 24. On the other hand, upon reproduction, if illumination reference light same as the reference light 200 used upon recording is illuminated on the hologram recording medium 24, then diffraction light corresponding to the recorded interference fringes is produced. Then, the diffraction light is condensed on the image sensor 26 by the lens 25 to form an image and photoelectrically converted by the image sensor 26. Thereafter, the resulting photoelectric conversion signal is processed to reproduce data.